<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="UTF-8">
  <meta http-equiv="X-UA-Compatible" content="IE=edge">
  <meta name="viewport" content="width=device-width, initial-scale=1.0">
  <title>Document</title>
  <style>
    .wrapper{
      font-size: 1em;
    }
    .box {
      border: 5px solid purple;
      padding: 10px;
      margin: 10px;
    }
  </style>
</head>
<body>
  <div class="wrapper">
    <div class="box" style="width: 200px;">我的宽度设置为200px</div>
    <div class="box" style="width: 10vw;">我的宽度设置为10vw</div>
    <div class="box" style="width: 10em">我的宽度设置为10em</div>
  </div>
</body>
</html>